This application claims the priority of Japanese Patent Applications No.11-19129 filed on Jan. 27, 1999, No.11-27467 filed on Feb. 4, 1999 and No.11-101706 filed on Apr. 8, 1999 which are incorporated herein by reference.
The present invention relates to a bearing mechanism, a hard disk drive mechanism and a polygon mirror drive mechanism using the bearing mechanism, and a method for forming herringbone groove portions of a dynamic-pressure bearing, more specifically, a method for forming herringbone groove portions on a bearing shaft or bearing sleeve of a dynamic-pressure bearing.
Conventionally, for example in hard disk drive mechanisms of storage devices, polygon mirror drive mechanisms of copying machines or laser printer devices and the like, dynamic-pressure bearings have often been adopted in order to achieve rotations accompanied by less swaying-rotations. As such a dynamic-pressure bearing, there has been known one, for example as shown in Japanese Patent Laid-Open Publication HEI 5-215128, in which a rotating shaft is inserted inside a cylindrical-shaped bearing member while, for example, herringbone-shaped dynamic-pressure generating grooves are formed circumferentially in the outer circumferential surface of the rotating shaft. In this structure, when the rotating shaft is rotated at high speed inside the bearing member, a radial dynamic pressure is generated in a gap between the rotating shaft and the bearing member by a pumping effect of fluid on the dynamic-pressure generating grooves. As a result, for example when a radial force acts on the axis of rotation due to vibrations or other disturbance, the dynamic pressure acts as a restoring force, thus allowing a stable rotation accompanied by less swaying-rotations to be realized.
In another aspect, ball bearings have conventionally been used as a bearing for motors or the like that serve for rotationally driving the disk in polygon mirrors of laser printers, hard disk drives or the like. However, since a periodic sways would occur due to errors of sphericity of the ball or the like, it has recently been practiced to use dynamic-pressure bearings instead of the ball bearings.
In such a dynamic-pressure bearing, which is designed to generate a dynamic pressure to the fluid present in a gap of the bearing by a pumping effect, spiral dynamic-pressure generating grooves such as herringbone grooves are formed in either one of the bearing shaft or the bearing sleeve to generate the dynamic pressure.
FIG. 11 is a sectional view schematically showing a configuration in which a conventionally generally known pneumatic dynamic-pressure bearing 110 having a vacuum pump function is applied to a polygon-mirror dedicated scanner motor of a laser printer. In the polygon-mirror dedicated scanner motor shown here, a bearing shaft 114 of the dynamic-pressure bearing 110 is mounted on a base 113 so as to be installed in a fixed and closed state at a central position in a housing 112, a bearing sleeve 115 is fitted around the bearing shaft 114 with a very slight gap, and a rotating mirror 118 is attached to this bearing sleeve 115.
Further, a magnet 120 magnetized to N and S poles is provided on an outer circumferential surface of the bearing sleeve 115, and a driving coil 116 as a motor is provided on an inner wall surface of the housing 112 so as to be opposed to the magnet 120. In this arrangement, about a few xcexcm deep V-shaped grooves 122, 122 . . . (herringbone grooves) are carved at regular intervals in the direction of rotation in the outer circumferential surface of the bearing shaft 114, where among these V-shaped grooves 122, 122 . . . , those carved in upper and lower portions of the bearing shaft 114 play a role of a vacuum pump and those carved at central portion of the bearing shaft 114 are purposed to support the bearing shaft 114 itself.
FIG. 12A shows an appearance view of the bearing shaft 114 of the dynamic-pressure bearing 110 shown in FIG. 11, and FIG. 12B shows a sectional view of the dynamic-pressure bearing 110 with the bearing sleeve included. As shown in FIGS. 12A and 12B, this bearing shaft 114 is internally formed into a hollow shape with one shaft end opened, gas inlet holes 124, 124 . . . for letting in gas (e.g. air) are provided at upper and lower portions of the bearing shaft 114, and gas inlet holes 125, 125 . . . for introducing gas (air in the atmosphere in this example) to around the bearing shaft 114 are provided between the base 113 of the bearing shaft 114 and the bearing sleeve 115.
In the dynamic-pressure bearing 110 constructed as shown above, when the rotating mirror 118 is rotated, air in the vessel is introduced through the gas inlet holes 125 into the bearing sleeve 115, then flowing on so as to be introduced into the hollow of the bearing shaft 114 via the gas inlet holes 124, 124 . . . of the bearing shaft 114 and discharged outside through the opening at one shaft end, as shown by arrows in the figure. Then, such a gas flow causes the internal pressure of the vessel to be reduced, enabling a smooth rotation without occurrence of a periodic sways or the like. When the rotating mirror 118 is stopped from rotating, the internal pressure of the vessel becomes equal to the external pressure.
Conventionally, groove machining by etching method has been used as a method for forming such dynamic-pressure generating grooves in the bearing shaft or the rotating mirror (bearing sleeve). This etching method includes masking in a specified configuration to form grooves at unmasked portions. Two types of methods are available to do this, one being wet etching which uses liquid phasexe2x80x94solid phase reaction with etchant and the other being dry etching which uses gas phasexe2x80x94solid phase reaction with reaction gas in plasma. Out of these two methods, the dry etching method is often used for groove formation by virtue of its relative superiority in machining precision. These etching methods are used for groove formation primarily in hard materials.
As another method for forming the grooves, rolling process has been used to form grooves of, for example, spiral or other shape. In this rolling process, material is sandwiched between dies or the like and, while being rotated by the dies or the like, plastically deformed, by which a specified configuration is formed. This rolling process is used for groove formation primarily in soft materials.
The formation of herringbone groove portions by the aforementioned etching method is largely affected in machining precision by the concentration of the etchant or the like. Also, the etchant erodes even inner circumferential surfaces of the herringbone groove portions, resulting in an unsatisfactory machining precision. Since configurational precision is required for herringbone groove portions to function as dynamic-pressure generating grooves, the etching method, which is low in machining precision, is unsuitable for the formation of herringbone groove portions. Furthermore, this etching method, which involves a totally large number of processes and moreover takes longer time for machining, is low in productivity and unsuitable for mass production, resulting in a poor practicability.
The rolling process, although suitable for machining of soft materials, is unsuitable for the formation of herringbone groove portions because dynamic-pressure bearings are made from hard material so as to withstand high-speed rotation. A hard material, if rolling processed, would tend to result in a considerable impairment of the shape of the dies or flaws of their surfaces. Deteriorations of the dies like this would cause the precision of the groove machining in a machining object to lower, and flaws of the surfaces of the dies would be transferred onto the surface of the machining object as they are, causing disadvantages that, for example, the surface of the machining object is unnecessarily cut out. This would result in an insufficient machining precision of dynamic-pressure generating grooves or the like in the process of machining the dynamic-pressure generating grooves in the bearing shaft or bearing sleeve of the dynamic-pressure bearing, so that a high-performance dynamic-pressure bearing could not be obtained.
On the other hand, in a mechanism using herringbone dynamic-pressure generating grooves, basically, enough radial dynamic pressure could not be generated unless the rotational speed is as high as 10-20 thousands of rotations in useless cases, thus making such problems as oil exhaustion due to the high-speed rotation more liable to occur. Also, in a low-rotation state upon a start-up or halt of the rotating mechanism, dynamic-pressure insufficiency that would inevitably be involved cause the bearing and the rotating shaft to come into contact with each other, making the members more liable to wear. Furthermore, although a dynamic-pressure bearing in which radial dynamic pressure is generated as a pneumatic pressure without using lubricating oil has also been proposed, yet use of pneumatic pressure would require further higher rotational speeds of 40-50 thousands of rotations to generate enough radial dynamic pressure according to discussions by the present inventors, in which case the aforementioned problems would matter to more extent. Therefore, besides improvement in the herringbone dynamic-pressure generating grooves, there is a desire for a mechanism capable of generating radial dynamic pressure with higher efficiency.
An object of the present invention is to provide a bearing mechanism which is capable of generating enough dynamic pressure even in lower-speed regions, and yet which can be easily surface-machined for the generation of radial dynamic pressure and manufactured with low cost, as well as a hard disk drive mechanism and a polygon mirror drive mechanism using the bearing mechanism. Another object of the invention is to provide a method for forming herringbone groove portions of a dynamic-pressure bearing which method is simple in manufacturing processes and low in manufacturing cost.
In order to solve the above objects, according to the present invention, there is provided a bearing mechanism comprising:
a first member of a shaft shape; and
a second member which has an insertion hole for the first member to be inserted through, and which forms, between an inner surface of the insertion hole and an outer circumferential surface of the first member, a bearing gap of a specified extent filled with a fluid while permitting the first member to rotate about an axis relative to the second member, wherein
in at least either one of the outer circumferential surface of the first member or an inner circumferential surface of the second member opposed thereto, dot-like minute dips and bumps are formed dispersedly, whereby the surface is roughened so that a center-line mean roughness of the surface is controlled within a range of 0.1 xcexcm-1.0 xcexcm, and wherein the first member and the second member are rotated relative to each other so that a radial dynamic pressure is generated around the first member in the bearing gap. In addition, the term xe2x80x9ccenter-line mean roughnessxe2x80x9d herein refers to a measurement obtained in compliance with a method defined JIS B0601.
Also according to the present invention, there is provided a hard disk drive mechanism comprising:
the bearing mechanism as defined above;
a driving section for, assuming that either one of the first member or the second member of the bearing mechanism is a fixed-side member and the other is a rotation-side member, driving the rotation-side member (hereinafter, referred to as rotating member) into rotation; and
a hard disk for recording use which is fitted to the rotating member and which rotates integrally therewith.
Also, according to the present invention, there is provided a polygon mirror drive mechanism comprising:
the bearing mechanism as defined above;
a driving section for, assuming that either one of the first member or the second member of the bearing mechanism is a fixed-side member and the other as a rotation side member, driving the rotation-side member (hereinafter, referred to as rotating member) into rotation; and
a polygon mirror which is integrated to the rotating member and which has a plurality of reflecting surfaces formed into a polyhedral shape so as to surround an axis of rotation of the rotating member.
Furthermore, according to the present invention, there is provided a method for forming herringbone groove portions of a dynamic-pressure bearing, comprising:
for forming herringbone groove portions in an outer circumferential surface of a bearing shaft of the dynamic-pressure bearing and an inner circumferential surface of a bearing sleeve into which the bearing shaft is inserted, masking the outer circumferential surface of the bearing shaft of the dynamic-pressure bearing and/or the inner circumferential surface of the bearing sleeve with a masking sheet having penetration holes of a desired herringbone configuration; and projecting small-diameter particles onto the masked surface by precision shot peening.
In the bearing mechanism of the invention, in at least either one (hereinafter, referred to as roughened surface) of the outer circumferential surface of the first member or the inner circumferential surface of the second member opposed to each other with the bearing gap therebetween, dot-like minute dips and bumps are formed dispersedly, by which the surface is roughened so that its center-line mean roughness Ra is controlled within a range of 0.1 xcexcm-1.0 xcexcm, thus enabling the bearing mechanism to generate enough radial dynamic pressure even in lower-speed regions. Still, because enough dynamic pressure can be generated at lower rotational speeds as compared with conventional bearing mechanisms using conventional dynamic-pressure generating grooves, the time duration of a dynamic-pressure insufficiency upon a start-up or halt of the rotating mechanism is shortened so that members in the bearing part can be made less liable to wear.
Also, the fluid of a specified amount to be filled in the gap between the outer circumferential surface of the first member and the inner circumferential surface of the second member opposed thereto may be either a gas or a liquid, where when the fluid to be filled is a gas, the need for lubricating oil is eliminated. Then, because no lubricating oil is used, the bearing mechanism can be made maintenance-free at least for oil supply. Besides, because the need for considering sealing for oil leak prevention, the structure can be simplified.
On occasions, for example, when some imbalance is present in the rotating shaft inserted inside the cylindrical bearing body or when some disturbance such as external force or vibrations has occurred in the radial direction, the rotating shaft may yield periodic or a periodic swaying-rotations. Among all types of swaying-rotations, for a periodic swaying-rotation, the position to which the rotating shaft shifts due to the swaying-rotation can be grasped as a definite pattern, so that the correction of the rotating shaft is possible. However, for swaying-rotation that occurs a periodically, because of random time and position of the occurrence, correction is impossible. Nonetheless, for the bearing mechanism of the present invention as described above, the eccentricity ratio of a periodic sway of the first member about the axis of rotation in the insertion hole can be made to fall within a range of 20% and under. This and other effects can be obtained similarly whichever the fluid to be filled in the bearing gap is a gas or a liquid.
In order to generate a uniform radial dynamic pressure, it is desirable that dot-like minute dips and bumps are formed so as to be generally uniformly dispersed two-dimensionally on the roughened surface facing the bearing gap, for example, as conceptually shown in FIG. 2 (reference numeral 2a denotes the roughened surface in this case). More specifically, for example when the center-line mean roughness is measured in arbitrary directions, for example arbitrary two directions perpendicular to each other, on the roughened surface, given measured values Ra1 and Ra2, it is desirable that the ratio of the absolute value |Ra1xe2x88x92Ra2| of a difference between the two measured values, to the mean value (Ra1+Ra2)/2, that is, (2xc3x97|Ra1xe2x88x92Ra2|/(Ra1+Ra2)), be within 30%.
For the bearing mechanism of the present invention, the relative number of revolutions of the outer circumferential surface of the first member and the inner circumferential surface of the second member for enough radial dynamic pressure to be generated is desirably controlled to approximately 2000 rpm or more. Number of revolutions less than 2000 rpm would cause insufficient generation of dynamic pressure, making the members more liable to wear due to increase in contact friction. Also, the bearing mechanism of the present invention, as described before, has a characteristic that enough dynamic pressure can be generated even at low-speed rotation regions (e.g., in a region of about 2000-20000 rpm, or a further lower region of about 2000-15000 rpm), as compared with conventional bearing mechanisms. However, the present invention is not limited to these ranges of number of revolutions, but well applicable also to bearing parts which rotate at speeds higher than the above ones, capable of producing unique effects such as prolonged lives of the members attributable to the friction reduction effect of the dynamic pressure.
As to the reason that the bearing mechanism of the present invention is capable of generating dynamic pressure at lower speeds than in conventional bearing mechanism making use of dynamic-pressure generating grooves, one factor could be that if the fluid filled in the bearing gap is a gas, the gas present in the bearing gap is hard to leak out of the gap during the relative rotation of the members by virtue of the minute dips and bumps that are dispersedly formed in at least either one of the outer circumferential surface of the first member or the inner circumferential surface of the second member opposed thereto, or in other words that the sealability between the outer circumferential surface of the first member and the inner circumferential surface of the second member is improved. Such an effect of sealable improvement is more remarkable when the center-line mean roughness is controlled within a range of 0.15 xcexcm-0.2 xcexcm. As a result, for example, shortage of radial dynamic pressure less occurs even when further higher-speed rotation is required, so that smooth, less-swaying rotations can be realized.
If the fluid filled in the bearing gap is a liquid, one factor could be the contribution of a wedge film effect that develops when the liquid present in the bearing gap between the outer circumferential surface of the first member and the inner circumferential surface of the second member is let into the gap narrowed by the bumps, by virtue of the minute dips and bumps that are dispersedly formed in at least either one of the outer circumferential surface of the first member or the inner circumferential surface of the second member opposed thereto. Whereas the wedge film effect is also produced by a so-to-speak macroscopic factor that the gap is locally narrowed by a relative decentering of the first member and the second member, yet a microscopic wedge film effect by the dips-and-bumps formation, it can be considered, acts to produce a multiplier effect so that the dynamic-pressure generating effect is further enhanced. Besides, since these dips and bumps are formed in a finer dispersion, as compared with conventional dynamic-pressure generating grooves, a wedge film effect of even more uniform, higher level can be expected.
With regard to the sealability improvement, on the other hand, since minute dips and bumps are formed in the surface facing the bearing gap, a remarkable labyrinth effect is generated when the liquid passes through between the dips and bumps of the bearing gap, so that the sealability between the outer circumferential surface of the first member and the inner circumferential surface of the second member is improved. Then, the sealability improving effect like this becomes more remarkable when the center-line mean roughness Ra is controlled within the range of 0.15 xcexcm-0.2 xcexcm. As a result, for example, shortage of radial dynamic pressure less occurs even when further higher-speed rotation is required, so that smooth, less-swaying rotations can be realized.
In addition, the concrete constitution that allows the radial dynamic pressure to be utilized as a restoring force for the axis of rotation can be exemplified by an embodiment in which the first member has a rotation supporting portion which is formed so as to be in contact with a supported portion formed on the second member side, and which rotatably supports the second member while permitting the second member to move radially within a range of the bearing gap.
Next, the dot-like minute dips and bumps to be formed in the roughened surface can be formed by projecting impact particles having a mean particle size controlled within a range of 5-100 xcexcm at a rate of 50 m/sec-300 m/sec onto the dips-and-bumps formation surface portion (a portion where the roughened-surface is formed). With this method, the surface roughening process for dispersedly forming the dot-like minute dips and bumps can be easily achieved, allowing a reduction in cost as compared with, for example, groove machining process by photo-etching or the like.
When particles harder than a material from which the dips-and-bumps formation surface portion is formed are used as the impact particles, the formation of the dips and bumps can be achieved efficiently. For example, when the material of the dips-and-bumps formation surface portion is an Fe-related material, the material of the hard particles may be ceramic particles such as silicon carbide, alumina, zirconia or silicon nitride, glass particles, or metal metallic particles such as high-speed tool steel or stainless steel (e.g., high-carbon stainless steel). In addition, there are some cases where the formation of dips and bumps is enabled even with use of impact particles that are not harder than the dips-and-bumps formation surface portion. For example, even with the use of impact particles that are not harder than the dips-and-bumps formation surface portion, if the impact particles have a 50% or more hardness of the surface portion, there are many cases where the formation of dips and bumps can be achieved.
As to the shape of the impact particles, it is particularly desirable to use spherical particles in order that minute dips and bumps are formed so as to be uniformly dispersed. In this case, use of spherical particles as uniform in size as possible is further advantageous in that the impact force can be uniformized. More specifically, given a mean particle size dm of the impact particles used and a standard deviation "sgr"d of a particle size d, it is preferable that the value of "sgr"d/dm is less than 0.05. In addition, whereas it is desirable to suppress variations in shape and dimensions of individual dips and bumps with a view to generating uniform radial dynamic pressure, it is effective to form the dips and bumps, for example, by iterating to a plurality of times the projection of the impact particles onto the dips-and-bumps formation surface portion.
Next, for the bearing mechanism of the present invention, from the viewpoint of sufficiently enhancing the radial-dynamic-pressure generating effect, given a radius r1 of the columnar-shaped outer circumferential surface of the first member and a radius r2 of the columnar-shaped inner circumferential surface of the second member, it is desirable that the value of r2xe2x88x92r1 is controlled within a range of 0.2-20 xcexcm. The value of r2xe2x88x92r1 being, as it were, a parameter that reflects the size of the bearing gap, if the value is less than 0.2 xcexcm, it becomes more likely that the outer circumferential surface of the first member and the inner circumferential surface of the second member come into contact with each other, making the members more prone to wear due to increase in friction in some cases. If the value of r2xe2x88x92r1 exceeds 20 xcexcm, on the other hand, the sealability of the gap is impaired, making the generation of dynamic pressure insufficient in some cases. More desirably, the value of r2xe2x88x92r1 is within a range of 4 xcexcm-10 xcexcm. In addition, r1 and r2 herein refer to values that are calculated as D1max/2 and D2min/2, respectively, where when diameter D1 or D2 of the outer circumferential surface or the inner circumferential surface, respectively, is measured with the measuring position changed, a measured maximum value of D1 is assumed as D1max and a measured minimum value of D2 is assumed as D2min.
Next, given a radius r1 of the outer circumferential surface of the first member, a radius r2 of the inner circumferential surface of the second member and a cylindricity C of each surface, it is desirable that an expression that Cxe2x89xa6(r2-r1)/2 is satisfied. If C is greater than (r2xe2x88x92r1)/2, then it becomes more likely that the outer circumferential surface of the first member and the inner circumferential surface of the second member come into contact with each other, making the members more prone to wear due to increase in friction in some cases. In addition, as the cylindricity herein referred to, a cylindricity defined in JIS (Japanese Industrial Standards) B0621, 5.4.
Next, the roughened surface having a center-line mean roughness unique to the present invention can be formed quite easily since the machining by the projection of hard particles as described above can be applied to, for example, the outer circumferential surface of the shaft-shaped first member. In this case, for the inner circumferential surface of the second member, at least the center-line mean roughness Ra is desirably controlled to 1.0 xcexcm or less from the viewpoint of avoiding the wear of members due to friction increase. Then, in order to further enhance the swaying-rotation prevention effect with respect to the axis of rotation by virtue of the radial dynamic pressure generation, it is even more desirable that the inner circumferential surface of the second member also be made into a similar roughened surface.
It is also possible that groove portions that contribute to the radial dynamic pressure generation are formed together with the dot-like minute dips and bumps as described above in at least either one of the outer circumferential surface of the first member or the inner circumferential surface of the second member opposed thereto. By adding such groove portions, the swaying-rotation prevention effect by the radial dynamic pressure generation can be further enhanced.
Next, the method for forming herringbone groove portions of a dynamic-pressure bearing according to the present invention is a method in which injection (projection) machining such as shot peening and shot blasting, which has conventionally been used for relatively coarse machining processes such as deburring or surface satin processing, is applied to the formation of herringbone groove portions, where the machining precision, which has been an issue in etching or rolling process, is dramatically improved by controlling the material, particle size and projection pressure of the small-size particles, and yet simple fabrication is enabled.
As the xe2x80x9csmall-size particlesxe2x80x9d in this case, particles of alumina, silicon carbide, glass beads, plastics or the like are preferably used. The particle size of the xe2x80x9csmall-size particlesxe2x80x9d is preferably within a range of 5-100 xcexcm, more specifically, 40-80 xcexcm. If the particle size is smaller than 5 xcexcm, it is difficult to form grooves having a expected depth. Also, if the particle size is larger than 100 xcexcm, fine configuration cannot be formed with dimensional precision.
Further, the projection pressure for xe2x80x9cprecision shot peeningxe2x80x9d may be selected appropriately depending on the materials of the projecting member and the projected member or the like, but is preferably within a range of 1-10 kg/cm2. If the projection pressure is smaller than 1 kg/cm2, it takes too much time to form the grooves of an expected depth. Even if a pressure larger than 10 kg/cm2 is applied, the depth of grooves that can be formed per unit area does not noticeably change, with the result that the load applied to the equipment is necessarily added.
Then, the equipment for xe2x80x9cprecision shot peeningxe2x80x9d is preferably one which is capable of projecting the xe2x80x9csmall-size particlesxe2x80x9d by controlling such conditions as projection pressure, projection amount per unit time, projection aperture of the nozzle, and the like, and also capable of quantitatively feeding the projection material (small-size particles).
Amount and time of projection per unit time may be selected as appropriate depending on the depth of formed groove portions. Further, projection aperture of the nozzle may be selected as appropriate depending on the particle size of the projection material and the configuration of the groove portions. xe2x80x9cMaskingxe2x80x9d may be implemented by using metal resist, photoresist, printing resist or the like. As the metal resist, a stainless thin plate, nickel electrocast products or the like may appropriately be used. As the photoresist, those superior in resistance to shocks are preferable, and ultraviolet-curing type urethane resin or the like may appropriately be used. As the printing resist, screen printing may appropriately be used.
In method for forming herringbone groove portions of a dynamic-pressure bearing according to the present invention, after masking the outer circumferential surface of the bearing shaft or bearing sleeve of the dynamic-pressure bearing, small-size particles are projected onto the outer circumferential surface of the bearing shaft or the inner circumferential surface of the bearing sleeve by precision shot peening, by which herringbone groove portions are formed. Therefore, the method involves no such complex procedures as etching, and yet takes shorter time for the formation of thee herringbone groove portions, thus very high in productivity and capable of mass production.
Further, according to the present invention, because of small particle size of the projection material and the capability of projection in an extremely narrow range by changing the projection pressure and the projection nozzle aperture, there is no need of precision masking, and herringbone groove portions having fine configuration can simply be formed.
The method for forming herringbone groove portions of a dynamic-pressure bearing according to the present invention is a method which enables the formation of herringbone groove portions to be achieved in short time quite conveniently and which is enabled to greatly reduce the cost required for the formation, whereas conventional methods would take long time through several steps of processes to achieve the formation. That is, the method of the invention is not only capable of making herringbone groove portions of fine configuration conveniently and with precision, but also highly useful industrially by virtue of its suitability to mass production.